1. Field of the Invention
The present invention relates to an exposing apparatus.
2. Related Background Art
Referring to the accompanying drawings, description will be made of a method for positioning an exposing unit 12xe2x80x2 relative to a photosensitive body in a conventional exposing apparatus.
FIG. 10 is a perspective view descriptive of a photosensitive drum 11xe2x80x2 and an exposing unit 12xe2x80x2. FIG. 11 is a diagram descriptive of a side surface.
Base members 23xe2x80x2 and 24xe2x80x2 are fixed to a front side plate 21xe2x80x2 and a rear side plate 22xe2x80x2 of a main body, respectively. Focusing pins 31xe2x80x2 and 32xe2x80x2 which serve as members for supporting the exposing unit 12xe2x80x2 are mounted on the base members 23xe2x80x2 and 24xe2x80x2 respectively so as to be movable in a depth direction (y direction).
Engaging portions of the base members 23xe2x80x2, 24xe2x80x2 and the focusing pins 31xe2x80x2, 32xe2x80x2 are threaded, whereby heights of the focusing pins 31xe2x80x2 and 32xe2x80x2 from the base members 23xe2x80x2 and 24xe2x80x2 are changed when the focusing pins 31xe2x80x2 and 32xe2x80x2 are turned. The exposing unit 12xe2x80x2 is mounted on two front and rear pedestals 31axe2x80x2 and 32axe2x80x2 of the focusing pins. When the focusing pins 31xe2x80x2 and 32xe2x80x2 are rotationally adjusted as described above, the pedestals 31axe2x80x2 and 32axe2x80x2 of the focusing pins are displaced in a height direction, whereby a position of the exposing unit 12xe2x80x2 is adjusted in the depth direction.
For locking and rattling prevention, a bonding agent is preliminarily coated over entire circumferences of the threaded portions of the focusing pins 31xe2x80x2 and 32xe2x80x2 which are engaged with the base members 23xe2x80x2 and 24xe2x80x2. Furthermore, tips of the focusing pins 31xe2x80x2 and 32xe2x80x2 are slotted so that these pins can be rotationally adjusted with a screwdriver.
Two positioning run-through holes are formed at both ends of the exposing unit 12xe2x80x2. On the other hand, shaft forms are disposed on the pedestals of the focusing pins 31xe2x80x2 and 32xe2x80x2. A round hole 121xe2x80x2 and an elongated round hole 122xe2x80x2 which are run-through holes are fitted over shaft root portions 31bxe2x80x2 and 32bxe2x80x2 of the focusing pins respectively. Accordingly, the exposing unit 12xe2x80x2 is positioned on a plane coordinate system (x-z coordinate system). Then, the exposing unit 12xe2x80x2 is fixed to the pedestals 31xe2x80x2a and 32xe2x80x2a of the focusing pins by urging the exposing unit using elastic fixing means (not shown) in a direction indicated by an arrow A in FIG. 11. Positioning and fixing of the exposing unit 12xe2x80x2 are thus completed.
Description will be made here of why the exposing unit 12xe2x80x2 is elastically fixed. An optical inconvenience such as curving of a scanning line may be produced when the exposing unit is deformed. When the exposing unit is fixed firmly with screws, the exposing unit may be deformed due to screw tightening torques during fixing or due to thermal expansion when a temperature rises in the exposing apparatus. When the exposing unit 12xe2x80x2 is mounted on the main body only by a weight of the exposing unit without being fixed, on the other hand, the exposing unit may be broken during transit or an image may be ununiform due to vibrations at an image forming time, whereby, the exposing unit 12xe2x80x2 is practically unusable. The exposing unit 12xe2x80x2 is therefore elastically fixed to the main body of the exposing apparatus to prevent the above described problems.
However, the above described conventional positioning method poses a problem that the method allows a depth of the exposing unit to be deviated as described below.
Fitting plays are reserved in a radial direction between the run-through holes 121xe2x80x2, 122xe2x80x2 and the shaft root portions 31bxe2x80x2 and 32bxe2x80x2 which are fitting parts. This is because the exposing unit cannot be mounted as a matter of course when no gap remains. Furthermore, the fitting plays serve not to restrict span changes on sides of the main unit and the exposing unit due to thermal expansion caused by the above described temperature rise in the exposing apparatus, thereby preventing a stress from being produced in the exposing unit.
Furthermore, axial lines of the front and rear focusing pins 31xe2x80x2 and 32xe2x80x2 are usually not in parallel with each other, but inclined due to tolerances of parts such as a main body frame. Fitting lengths are therefore set rather short so that the exposing unit 12xe2x80x2 can be mounted.
No optical problem is posed even when the exposing unit is statically deviated in the x direction and the z direction within the range of the fitting play. However, inclination of the exposing unit poses a problem of a deviation (inaccuracy) of a depth. A cause for the depth deviation is classified into (1) tilting of the exposing unit and (2) poor reproducibility of a positional relation between a jig and the exposing unit.
First, description will be made of xe2x80x9cdepth deviation due to tilting of the exposing unitxe2x80x9d. When an external force is exerted to the exposing unit, the exposing unit is inclined, thereby changing an optical path length. A concrete example of exerted external force is a stress produced by an electric line bundle (between the exposing unit and the main body) or the like.
Referring to FIGS. 12A, 12B, 13A and 13B, description will be made of the tilting of the exposing unit and the change of the optical path length. FIG. 12A is a sectional view of the focusing pin 31xe2x80x2 of the exposing unit 12xe2x80x2 and FIG. 12B is an optical diagram in a regular condition corresponding to FIG. 12A. Furthermore, FIG. 13A is a diagram showing a condition where the exposing unit is inclined and FIG. 13B is an optical diagram in an inclined condition corresponding to FIG. 13A.
Rays emitted from light emitting means 201xe2x80x2 in the exposing unit 12xe2x80x2 are imaged by a lens array 202xe2x80x2 used as imaging means onto a surface of the photosensitive drum 11xe2x80x2 which is an electrophotographic photosensitive body. A regular optical path length L1 shown in FIG. 12B is 10 mm.
On the other hand, a diameter d of the run-through holes 121xe2x80x2, 122xe2x80x2 and the shaft root portions 31bxe2x80x2, 32bxe2x80x2 has a nominal value of 4 mm, and the fitting plays have a diameter of 20 xcexcm. A fitting length b is set at 1.6 mm. Furthermore, the pedestal 31axe2x80x2 has a radius R1 of 4.5 mm. When the exposing unit 12xe2x80x2 is inclined relative to the focusing pins, an inclination angle "THgr"1 is 0.7 degrees at maximum.
In FIGS. 13A and 13B, an external force is exerted in a direction indicated by an arrow B. The exposing unit 12xe2x80x2 is inclined around a point C in contact with the pedestal 31axe2x80x2 of the focusing pin which functions as a rotating fulcrum. In the inclined condition, an optical path length L2 is 10.06 mm. That is, a depth deviation of 60 xcexcm is produced.
This depth deviation is geometrically reduced by shortening a radius R1 of the pedestal 31axe2x80x2. However, the radius cannot be shortened easily since the shortening of the radius produces a defect to make a mounted condition of the exposing unit dynamically unstable, whereby an image may be uneven (ununiform) due to the vibrations.
Then, description will be made of xe2x80x9cdepth deviation due to poor reproducibility of the positional relation between the jig and the exposing unitxe2x80x9d. FIG. 14 is a diagram descriptive of a mounted condition of jig units 99xe2x80x2 relative to the focusing pins 31xe2x80x2 and 32xe2x80x2, and FIG. 15 is a diagram descriptive of a mounted condition of the exposing unit 12xe2x80x2 relative to the focusing pins 31xe2x80x2 and 32xe2x80x2. A section of a front side of the exposing unit 12xe2x80x2 is shown on a left side and a section on a depth side is shown on a right side in FIG. 14.
Heights of the focusing pins 31xe2x80x2 and 32xe2x80x2 are usually adjusted with the Jig units 99xe2x80x2 having dial gauges or the like mounted on portions to be adjusted (focusing pins) in the main body. After the adjustment, the jig units 99xe2x80x2 are dismounted and the exposing unit is assembled.
On the other hand, the axial lines of the front and rear focusing pins 31xe2x80x2 and 32xe2x80x2 are not in parallel with each other under influences of the allowances of the parts as described above. Surfaces of the pedestals of the focusing pins 31xe2x80x2 and 32xe2x80x2 have an inclination angle which is indicated by "THgr"2 in FIG. 14. Since the pedestals on which the exposing unit is to be mounted do not form a planar surface and the fitted portions have a degree of freedom, a posture (angle) of the exposing unit is not defined clearly but optional. Description will be made below of a mechanism to produce the depth deviation when a mounted condition (the posture of the exposing unit) is changed between adjusting time with the jig units and an exposing unit assembling time. The inclination angle "THgr"2 is assumed as 0.5 degree.
The jig units 99xe2x80x2 are assumed to be along the surface of the pedestal 31axe2x80x2 of the front side focusing pin 31xe2x80x2 at the adjusting time with the jig units as shown in FIG. 14. At this time, a depth side is supported at a point. The jig unit 99xe2x80x2 is brought in contact with a portion D which is a corner of the surface 32axe2x80x2 of the focusing pin 32xe2x80x2. The heights are adjusted with the exposing unit kept in this posture. Both front and rear imaginary optical path lengths Lf1 and Lr1 are 10 mm.
In contrast, let us consider a case where mounted conditions of the jig units are not reproduced due to the posture of the exposing unit. The exposing unit 12xe2x80x2 is assumed to be along the surface of the pedestal 32axe2x80x2 of the depth side focusing pin 32xe2x80x2 as shown in FIG. 15. The front side is in contact with a portion E which is a corner of the surface of the pedestal 31axe2x80x2 of the focusing pin 31xe2x80x2. The exposing unit 12xe2x80x2 shown in FIG. 15 is rotated clockwise relative to the jig units 99xe2x80x2 shown in FIG. 14. As the exposing unit is rotated, the front side floats up using the portion E as a fulcrum, whereas the depth side sinks using the portion D as a fulcrum. Accordingly, the optical path length is changed. The front and rear optical path lengths Lf2 and Lr2 have deviation distances of +40 xcexcm and xe2x88x9240 xcexcm respectively from the regular optical path length.
As causes for degrading a reproducibility of the mounted conditions, there can be mentioned (1) an external force exerted (example: a stress applied to the units from an electric line bundle), (2) slight differences in forms, masses and centers of gravity of the jig units from those of product units, and (3) randomness (contingency) due to stabilities of the mounted conditions which are at a similar degree.
The depth deviation on the order of 60 xcexcm at maximum can be produced due to xe2x80x9ctilting of the exposing unitxe2x80x9d and xe2x80x9cpoor reproducibility of the positional relation between the jig units and the exposing unitxe2x80x9d as described above.
Then, description will be made of a standard (tolerance) for the depth. The depth is generally on the order of xc2x170 xcexcm though the depth is dependent on a design of an optical system. This value is classified dependently on tolerances as described below.
(1) Tolerance for adjustment in the exposing unit: xc2x120 xcexcm
(2) Tolerance for adjustment in the main unit (between a drum support portion and a exposing unit attaching portion): xc2x120 xcexcm
(3) Precision for parts of the photosensitive drum (swing of the photosensitive drum): xc2x120 xcexcm
(4) Margin: xc2x110 xcexcm
Items (1) through (3) mentioned above are critical for steps of mass production, thereby resulting in only a little margin.
The above described depth deviation distance (60 xcexcm) is far larger than the margin (10 xcexcm), thereby making a depth out of the standard. As a result, the depth deviation distance produces a defective image which is defocused.
As compared with a laser scanning system, an LED exposing system which forms a latent image on an image bearing body by imaging rays emitted from light emitting means composed of a plurality of light emitting diodes (LEDs) on the image bearing body by imaging means in particular has a merit to configure an apparatus remarkably compact. The LED exposing system exhibits a remarkable effect in particular for a color image forming apparatus which comprises a plurality of image forming portions. A slight inclination of the exposing unit therefore produces a large influence on an image quality.
Accordingly, an LED array which is the light emitting means and a SELFOC lens array which is the imaging means are usually configured as a unit in the LED exposing system, but for controlling an optical path length within a predetermined range, it is necessary to adjust a position of an LED head used as the exposing unit (exposing means) relative to the photosensitive body with a precision on the order of some tens of microns in the depth direction.
An object of the present invention is to provide an exposing apparatus which prevents an exposing unit from being inclined, thereby preventing images from being defocused.
Another object of the present invention is to provide an exposing apparatus which allows an exposing unit to be mounted easily and with a high precision.
Still another object of the present invention is to provide an exposing apparatus which comprises an exposing unit for exposing a photosensitive body, a first fixing member for positioning and fixing a vicinity of an end of the exposing unit on a side of the above described photosensitive body, and a second fixing member for fixing a vicinity of an end on a side opposite to a fixing position fixed by the first fixing member.
Further objects of the present invention will be apparent from the following description.